This invention relates to an accumulator fuel injection apparatus applied to internal combustion engines, such as a diesel engine.
The conventional fuel injection apparatuses for multi-cylinder engines include an apparatus of a fuel injection system (electronically controlled fuel injection system) in which the controlling of an injection rate and injection time is done by an electronic circuit, an apparatus of a common injection system (common-rail injection system) in which a fuel is distributed from an injection pump to combustion chambers through a common passage, and an apparatus of a pressure storage type injection system (accumulator injection system) in which a fuel is distributed from an injection pump to combustion chambers through a common passage and an accumulator. Since the fuel injection apparatuses themselves of these systems are not provided with an accumulator in which the fuel from an injection pump is temporarily stored, the supplying of the fuel to these apparatuses is done through a common rail, a common passage, i.e. an accumulator.
FIG. 8 shows an injector (which will hereinafter be referred to as a first conventional example) for a conventional accumulating fuel injection apparatus. Such a conventional injector is a pressure balancing type injector disclosed in, for example. Japanese Patent Laid-Open Nos. 165858/1994 and 282164/1987, which is formed so that a fuel is supplied to or discharged from a balancing chamber by turning on or off a solenoid valve, whereby a needle valve is seated on or lifted from a seat of the nozzle, and which is adapted to lift the needle valve from the seat by removing a needle valve closing fuel pressure applied to the interior of the balancing chamber, whereby the injection of the fuel is carried out. Such a structure will now be further described. A casing 31 of an injector 30 is provided therein with a guide bore 32, a fuel storage chamber 33 and a control volume, i.e. a balancing chamber 32. A needle valve 35 is provided slidably in the guide bore 31 The needle valve 35 comprises a larger-diameter portion 36, and a smaller-diameter portion 37 integral with the larger-diameter portion 36, and a needle 38 is provided on a lower end of the smaller-diameter portion 37. The casing 31 is provided with a hole type injection nozzle 39 (refer to FIG. 11), and the injection nozzle 39 has injection holes 40 at a lower end portion thereof. The injection nozzle 39 is also provided with a seat 41 on an inner surface of its lower end portion, and, when the needle 38 of the needle valve 35 sits on the seat 41, the injection holes 40 are closed. In the hole type injection nozzle 39, the fuel collected in a passage, which extends from the seat 41 to a combustion chamber, after the valve is closed is ejected (after-dripping) in some cases due to the high temperature and pressure variation in the combustion chamber, and the fuel becomes an unburnt gas to cause the HC in an exhaust gas to increase. Therefore, it is necessary that the volume (sack volume 49) of a space extending from the seat 41 to the injection ports 40 be set as small as possible.
The casing 31 has a supply port 42 for introducing a high-pressure fuel from an accumulating pipe (not shown) into the interior thereof, and a flow passage communicating with this supply port 42 branches into two flow passages 43, 44, one flow passage 43 communicating with the balancing chamber 34 via an orifice B, the other flow passage 44 communicating with the fuel storage chamber 33. The casing 31 further has an orifice A allowing communication of the balancing chamber 34 with the outside.
The casing 31 is provided with a solenoid valve 45 for opening and closing the orifice A. The high-pressure fuel introduced from the supply port 42 enters the balancing chamber 34 and fuel storage chamber 33 and works on the needle valve 35. When the solenoid valve 45 is in an OFF-state, the orifice A (discharge passage 46) is closed therewith. In the meantime, the high-pressure fuel is supplied to the balancing chamber 34 and fuel storage chamber 33, so that the needle valve 35 is pressed against an inner lower surface of the injection nozzle due to a difference in the areas on which a pressure is exerted of the needle valve 35 with the injection ports 40 thereby put in a closed state. When a solenoid 47 of the solenoid valve 45 is excited, a valve disc 48 is attracted thereto, and the orifice A is opened, so that the pressure in the balancing chamber 34 decreases. When a needle valve lifting force based on the pressure in the fuel storage chamber 33 becomes larger than a needle valve lowering force based on the pressure in the balancing chamber, the needle valve 35 moves up, and the injection holes 40 are opened, the injection of the fuel starting. When the solenoid 47 of the solenoid valve 45 is then deenergized, the valve disc 48 closes the orifice A, and the fuel pressure in the balancing chamber 34 increases instantly by the high-pressure fuel introduced through the orifice B. Consequently, the needle valve 35 lowers, and the injection ports 40 are closed, the injection of the fuel stopping. When the orifice A is dosed by putting the solenoid valve 45 in an OFF-state, to instantly increase the fuel pressure in the balancing chamber 34, a flow of the fuel leaving the fuel storage chamber 33, passing through the injection nozzle 39 and injected from the injection ports 40 occurs, and, therefore, the fuel pressure becomes gradually low toward the lower end of the injection nozzle 39 due to the resistance of an annular fuel flow passage formed between the smaller-diameter portion 37 of the needle valve 35 and the portion of an inner surface of the casing 31 which is around the same portion 37 of the needle valve. Accordingly, a generally lowering force is exerted on the needle valve 35 on the basis of the high fuel pressure in the balancing chamber 34, the fuel pressure in the fuel storage chamber 33 and the fuel pressure on the seat 41, so that the needle valve 35 is closed.
FIG. 9 is a schematic diagram showing a fuel supply system in a conventional accumulator fuel injection apparatus. The orifices A, B are fixed orifices (the inner diameters dA.dB of the orifices A, B are constant), and the orifice A is set larger than the orifice B (dA greater than dB). Accordingly, a flow rate of a fuel flowing out from the orifice A is determined by the size of the orifice B. The lift of the needle valve 35 attains a peak when an injection rate is not lower than a certain level.
FIG. 10 is a graph showing the relation between the area characteristics of injection holes of an injector used for a diesel engine, i.e. the lift of a needle valve 35 in the injector and an effective opening area of an injection nozzle 39. Although when the lift is low, i.e., when the lift of the needle valve 35 is low, the effective opening area of the injection nozzle 39 increases in accordance with the size of a clearance between a needle 38 and a seat 41, when the area of the clearance exceeds that of the injection ports 40, the effective opening area becomes constant irrespective of the lift of the needle valve 35.
A conventional example shown in FIG. 12 is an example (which will hereinafter be referred to as a second conventional example, in which the structural elements equivalent to those of the first conventional example are designated by the same reference numerals, whereby repeated detailed descriptions of the elements are omitted), in which a return spring 52 for exerting a lowering force on a needle valve 35 is provided so that an effect in closing the needle valve 35 is obtained more reliably not by depending upon the flow passage resistance alone when a solenoid valve is in an OFF-state. The needle valve 35 in the second example comprises a larger-diameter portion 36, a smaller-diameter portion 37 and a diameter-reduced portion 50 formed in the larger-diameter portion 36. The return spring 52 is held in a low-pressure portion 51 formed between a casing 31 and the diameter-reduced portion 50. The end portion of the return spring 52 which is on the side of the larger-diameter portion 36 is engaged with a spring seat 53 supported on a shoulder portion, which is in the low-pressure portion 51, of the casing 31, while the end portion of the return spring 52 which is on the side of the smaller diameter portion 37 is engaged with a spring seat 54 supported on a lower shoulder portion of the diameter reduced portion 50. The return spring 52 constantly urges the needle valve 35 in the closing direction, and has an effect in preventing the after-dripping of the fuel from an injection nozzle by speedily carrying out the closing of the needle valve 35. The fuel leaking out into the low-pressure portion 51 is recovered by a fuel tank through a flow passage 55. A flow passage 43 extending from a supply port 42 communicates with a balancing chamber 34 via a flow passage 56, which is formed in the larger-diameter portion 36, and an orifice C (corresponding to the orifice B in the Conventional example shown in FIG. 8, and having a diameter d ) Even when a sufficient valve-closing effect cannot be obtained with a valve closing force with which a fuel pressure works on the needle valve 35 and a valve opening force balanced with each other, the return spring 52 closes the needle valve 35 reliably.
The performance level with respect to the fuel consumption, output horsepower and exhaust gas which is required for an engine in recent years has increased. In order that an engine meets a high level of various kinds of performance, it is demanded that an amount of a fuel injected per unit time from injection ports, i.e. a fuel injection rate be controlled finely in accordance with conditions such as an engine load. As the basic techniques for meeting the demand, it is necessary to enable the lift of a needle valve to be controlled at least in a plurality of stages.
The controlling of a fuel injection rate in an initial stage of fuel injection, i.e. an initial injection rate may be given as an example of a fine fuel injection rate controlling operation. When an initial injection rate is high, combustion noise and NOx occur.
In order to carry out an optimum fuel injection rate control according to the engine speed and the load condition, it is necessary that the lift of the needle valve can be controlled accurately, i.e., a half lift control operation for retaining the needle valve in a half lifted state can be carried out. However, the injectors as in the first and second conventional examples are adapted to fully lift or seat the needle valve 35 from or on the seat 41 by operating the solenoid valve on or off, and they are not so formed that a half lifted condition can be precisely controlled.
Another injector (which will hereinafter be referred to as a third conventional example) in which the controlling of an initial injection rate is done by employing a mechanism capable of varying the number of injection ports has been proposed (refer to, for example, Japanese Utility Model Laid-Open No. 142170/1982).
In a hole type injection nozzle 39 shown in FIG. 11, a distance d between a needle 38 and a seat 41 is small when the lift is low (in a position of solid lines), and, therefore, the seat 41 in a fuel injection passage extending from a supply port 42 to injection ports 40, from which the fuel is injected, via a fuel storage chamber 33 constitutes the largest restriction. When the needle valve is fully lifted (in a position of broken lines), the opening area at the seat 41 is larger than that of the injection ports, so that the effective opening area is naturally determined by the opening area of the injection ports 40. However, when the lift is low, the opening area at the seat 41 is smaller than that of the injection ports 40, so that the effective opening area is determined by the opening area at the seat 41. Therefore, when the lift is low, the pressure of the injected high-pressure fuel, i.e. a fuel pressure P2 becomes lower than that (common rail pressure) P1 working on the needle valve 35 (P2 less than P1). Namely, the actual injection pressure P2 produced when the lift is low becomes lower than a required injection pressure P1, i.e., low-pressure injection is carried out. Consequently, the atomization of the fuel is not attained, and smoke increases.
As shown in FIG. 13, a variable-number-of-injection-port mechanism 12 has a plurality of injection ports 14a, the diameter of which is smaller than that of the conventional injection ports 40, in a cylindrical portion 13 formed at a lower end part of an injection nozzle 11, the injection ports 14 being arranged in the direction (refer to an arrow C) in which a needle valve 6 is lifted. These injection ports 14a are formed so that a total opening area thereof becomes larger than that of the conventional injection ports. Since the injection ports 14a are formed so that they are all closed at an outer circumferential surface 6a of the needle valve 6 when a needle 9 of the needle valve 6 engages a seat 15, the after-dripping rarely occurs. The needle valve 6 is provided at a lower end portion thereof with an oil feed port 16, which communicates with a passage 18 formed in a diameter-reduced portion 17 of the needle valve 6.
According to the variable-number-of-injection-port mechanism 12, when the needle valve 6 is lifted, a fuel storage chamber 4, passage 18 and oil feed port 16 communicate with one another, and the closed injection ports 14a are opened sequentially in accordance with the lift of the needle valve 6. For example, when the lift of the needle valve 6 is S1, the lower injection ports 14a only are opened, and, when the lift of the needle valve 6 is S2, not only the lower injection ports 14a but also the upper injection ports 14a are opened. Therefore, according to the variable-number-of-injection-port mechanism 12, the opening area of the opened injection ports 14a in an initial stage in which the lift of the needle valve is low is smaller than that of the conventional injection ports in the same condition, so that an initial injection rate can be minimized.
The mechanism 12 is also suitably used when the pilot injection is carried out. In a fuel injection apparatus adapted to inject a fuel, which is required for one combustion of an internal combustion engine, in a plurality of shots, the injection (pilot injection) of a very small amount of fuel is carried out in some cases when a fuel ignition delay has to be prevented, prior to the main injection in which a greater part of the fuel is injected. The mechanism 12 is suitably used when such pilot injection is carried out.
In the injector of the third conventional example provided with a mechanism 12, the opening area of each injection port 14a is smaller than that of each injection port 40 of the first conventional example. Accordingly, even when the lift of the needle valve is low, the effective opening area is determined by the opening area of the injection ports 14a, and the initial injection rate can be controlled to be low. However, in the injector of the third conventional example, it is necessary that the half lift condition of the needle valve 6 can be controlled. Therefore, it is impossible to use this injector in combination with the injectors of the first and second conventional examples in which the half lift condition of the needle valve cannot be controlled.
The pressure balancing type injectors in which the controlling of the half lift condition of a needle valve is done include an injector (which will hereinafter be referred to as a fourth conventional example. Refer to, for example, Japanese Patent Laid-Open No. 161165/1990) in which the resilient force of return springs of different loads is exerted on the needle valve in order, whereby a half lift condition of the needle valve is temporarily created. The needle valve is formed of a smaller-diameter piston and a larger-diameter piston, and the pilot injection can be carried out by the lifting of the smaller-diameter piston prior to the main injection based on the lifting of the larger-diameter piston.
The means for half lifting a needle valve include a means for exciting a solenoid valve for only a very short period of time (which will hereinafter be referred to as a fifth conventional example. Refer to, for example, Japanese Patent Laid-Open No. 159184/1994.). This means is adapted to shut off a solenoid valve so as to close a discharge passage as soon as this passage is opened by energizing the solenoid valve. Owing to such a control operation, a fuel pressure is applied to a balancing chamber with the needle valve in a half lifted state before the needle valve is fully lifted, to cause the needle valve to be seated.
However, in the fourth and fifth conventional examples, the half lifted state of the needle valve cannot be retained, though the half lifted state can be temporarily obtained. Moreover, when such a half lifting means is used, the lift of the needle valve scatters due to the influence of the actual fuel pressure, so that it is difficult to precisely control the half lifted state of the needle valve. In addition, the on and off control of the solenoid valve have to be repeated in a short period of time. Therefore, a high-performance solenoid magnetic valve is required, and this causes the manufacturing cost to increase.
Therefore, an object of the present invention is to solve these problems, and provide a pressure balancing type accumulating fuel injection apparatus capable of controlling the lift of a needle valve precisely, retaining a half lifted state of the needle valve, and satisfying the requirements for a high performance level of the apparatus with respect to an engine developed in recent years.
Another object of the present invention is to provide an accumulating fuel injection apparatus formed so that the half lifted state of a needle valve can be controlled precisely, and capable of controlling an initial injection rate which allows the minimization of the occurrence of combustion noise and emission of HC and NOx to be attained.
The present invention relates to an accumulating fuel injection apparatus having a needle valve adapted to open and close an injection nozzle provided with injection ports in a lower portion thereof, a balancing chamber adapted to apply a fuel pressure to the needle valve, a supply passage for supplying a fuel from a fuel supply port to the balancing chamber, a discharge passage for discharging the fuel from the balancing chamber, a solenoid valve adapted to open and close the discharge passage, and a lift control means for controlling the lift of the solenoid valve, characterized in that the lift of the solenoid valve is increased and decreased by a control operation of the lift control means, an opening area of the discharge passage being increased and decreased in accordance with the lift of the solenoid valve, the opening area of the supply passage and the degree of opening of the injection nozzle being increased and decreased in accordance with the lift of the needle valve.
The solenoid valve acts as a variable lift valve.
In this accumulating fuel injection apparatus, the lift of the solenoid valve can be controlled, so that an opening area of the discharge passage, i.e. an amount of discharge per unit time of the fuel from the balancing chamber can also be controlled in a stepped manner. This enables the controlling of an amount of the fuel flowing into the balancing chamber through the supply passage of a predetermined opening area to be done so that this amount corresponds to the mentioned amount of discharge, i.e., the controlling of the lift of the needle valve which determines the opening area of the supply passage to be done as well. Accordingly, the degree of opening of the injection nozzle opened and closed with the needle valve, i.e. the injection rate of the fuel from the injection nozzle can be controlled with a high accuracy. Moreover, the half lifted condition of the needle valve can be retained by an operation of the solenoid valve, and the controlling of the fuel injection time can also be done easily.
The lift control means is adapted to deenergize or energize the solenoid, whereby the control means can be used as a stopper limiting the motion of the valve disc of the solenoid valve in at least two positions. In this case, the stopper limits the motion of the valve disc of the solenoid valve in at least two positions by a simple method, i.e., the deenergization or energization of the solenoid, and the fuel injection rate can thereby be controlled in at least two stages, i.e., at higher and lower levels.
When a groove type passage formed between the needle valve and a valve casing, which is adapted to guide the needle valve slidingly, is included in the supply passage, the opening area of an orifice at which the groove type passage faces the balancing chamber increases and decreases in accordance with the lift of the needle valve, so that the lift of the needle valve can be controlled accurately and stably.
The degree of opening of the injection nozzle may be controlled in accordance with the lift of the needle valve away from the seat in a position just on the upstream side of the injection ports and the opening area of the injection ports adapted to be opened by the needle valve, or in accordance with the number of injection ports actually opened by the needle valve when the injection ports comprise a plurality of rows of injection ports. Accordingly, the degree of opening of the injection nozzle is low when the lift of the needle valve is low, and becomes highest when the needle valve is fully lifted.
When the lift of the needle valve in the accumulating fuel injection apparatus and an engine load are set correlative, the fuel injection rate in a low-load condition can be set low by reducing the opening area of the discharge passage, and that in a high-load condition can be set high by increasing the opening area of the discharge passage.
When the fuel passage, which extends to the injection ports formed at a lower end portion of the injection nozzle, in the accumulator fuel injection apparatus has a flow passage resistance high enough to lower the fuel pressure when a fuel flow exists, a force working on the needle at the lower end portion of the needle valve to lift the needle valve can be reduced at such time that equal fuel pressure is applied to the balancing chamber and injection nozzle by closing the discharge port with the solenoid valve deenergized. This enables the closing of the needle valve to be done reliably.
When a return spring urging the needle valve in the closing direction thereof is provided between the needle valve and casing in this accumulating fuel injection apparatus, the needle valve receives, when the discharge port is closed by deenergizing the solenoid valve, a high fuel pressure occurring momentarily in the balancing chamber, a fuel pressure in the fuel storage chamber and a fuel pressure occurring on the seat in accordance with the respective pressure receiving surface area. Even when a difference between a force based on a fuel pressure and working in the valve closing direction and a force based on the fuel pressure and working in the valve opening direction is small, so that a sufficiently large valve closing force cannot be obtained, the needle valve can be closed reliably since the return spring urges the needle valve constantly in the valve closing direction. When the discharge passage is opened by energizing the solenoid valve, the fuel is discharged from the balancing chamber whether the needle valve is half lifted or fully lifted. Therefore, the pressure in the balancing chamber lowers, and the injection ports are opened by the needle valve. Owing to the positive urging force in the valve closing direction of the return spring, a speedy valve closing action of the needle valve can be obtained, and the after-dripping of the fuel can be prevented.